High temperature processing of silicon wafers is important for manufacturing modern microelectronics devices. Such processes, including silicide formation, implant anneals, oxidation, diffusion drive-in and chemical vapor deposition (CVD), may be performed at temperatures ranging from about 300° C. to 1200° C. in either multi-wafer batch furnaces or in single-wafer rapid thermal processors. These steps typically require a very accurate control of the temperature, pressure, and flow of gases in the chamber. In one embodiment of the prior art, the processing steps are done in a tube furnace, where wafers are processed in batch mode for increased throughput.
In conventional batch furnaces of the prior art, electrical heating elements are typically used. Electrical heating elements such as metal coil elements are usually insulated with a ceramic material, e.g., sintered alumina or aluminum nitride, inherently susceptible to mechanisms generating cracks and particles when undergoing thermal cycling. The thermal mass of such systems is high and results in low heating and cooling rates. Moreover, due to the discontinuous surface of conventional coil elements, the power density varies dramatically across the area to be heated. Additionally, because of the differential thermal expansion between the metal heating elements and the ceramic insulation during thermal cycling, the use of heaters in the prior art can result in particle generation or failure. During processing procedures while power is applied, the particles do not generally cause processing problems, as they are typically suspended above the semiconductor substrate. However, as the power is reduced at the end of a processing cycle, the forces that suspend the particles dissipate allowing them to fall and land upon the semiconductor substrate surface causing contamination. The falling action can be the result of both gravity and electrostatic attraction to the semiconductor substrate. Besides the particle contamination problem with conventional batch furnaces employing heaters comprising sintered ceramic materials, there is also a reliability issue and inherent limitations in terms of heating and cooling rates. Sintered ceramics are susceptible to thermal shock and tend to break when undergoing high temperature gradients.
In order to reduce the problems of unwanted dopant diffusion or particles from heaters landing on the semiconductor substrate surface causing contamination, high intensity lamps are used in some conventional semiconductor batch processing chambers. Radiant lamps allow very fast heating because of their extremely low thermal mass and rapid cooling because they can be turned off instantly. However, problems may be encountered with the use of high intensity lamps because they are localized energy sources. Not only do temperature differences arise during heating and cooling transients, as in traditional batch furnaces, but non-uniformities may also persist during processing. The interior walls of typical lamp based RTP systems are usually relatively cool and are not heated to a uniform equilibrium process temperature as in a conventional batch furnace. For larger diameter wafers, it may be difficult to maintain a uniform temperature across a wafer. For other systems, they fail to provide uniform wafer-to-wafer heating to a boat of multiple wafers. Many lamps use a linear filament, which makes them ineffective at providing uniform heat to a round wafer. It may be necessary to dynamically detect temperature non-uniformities and actively adjust heating during processing. This, in turn, may require complex temperature measurement systems. Additional problems may be encountered in some lamp based systems due to aging and degradation of lamps and other components. As a result, it may be difficult to maintain repeatable performance and frequent replacement of parts may be necessary.
Besides the contamination and uniformity issues, there is a conformity limitation in batch systems of the prior art. Batch systems for semiconductor wafer processing often have a cylindrical geometry, which is driven by the round shape of the wafers. The geometry of the heating system often tries to conform to the geometry of the substrates. In the resistive heaters of the prior art, metal coil elements embedded in ceramic insulation do not provide a continuous heating surface. Systems employing arrays of lamps typically cannot provide a cylindrical heating surface.
The invention relates to a semiconductor processing system which can provide fast, uniform, energy-efficient, and contaminant-free heating and cooling to a plurality of wafers, while allowing the design flexibility desirable in batch processing apparatuses.